1. Field of the Invention
The invention relates to a shape meter for measuring deformations in sheets of magnetic material during cold-rolling.
In the cold rolling of thin metal strips, the strip may appear quite flat when being rolled up on the coil under high tensile stress, even if, during the subsequent recoiling under low tensile stress, it displays local waviness either at the edges or in the middle, but mainly at the edges. In the latter case, with edge waves having the wave length L, these are apparently longer than the flat middle portion of the same sheet, and the relative length difference .DELTA.L/L is a measure of the off-flatness.
Since the tensile stress in the strip in a cold rolling mill is usually so high between the last roll pair and the coil that the strip at that point is quite flat, the length difference .DELTA.L/L is transformed into a stress difference .DELTA..sigma. = E (.DELTA.L/L), where E is the modulus of elasticity of the strip material. If .DELTA..sigma. can be measured when the strip is tautly stretched, it will be possible to calculate the off-flatness of the material in the free condition.
In magnetic materials the distribution of the stresses can be measured in a strip which is elastically flattened under tensile stress by utilizing the magnetic anisotropy generated in the strip by the tensile stress. In material with positive magnetostriction, such as steel at a moderate magnetization, the permeability increases in the drawing direction and decreases in the cross-direction.
This anisotropy can be sensed in a contact-free manner by means of a transducer of the type shown and described in U.S. Pat. No. 2,912,642. Such a transducer comprises two iron cores arranged perpendicularly to each other and provided with windings, one of the iron cores being excited by alternating current and inducing a magnetic field in the measuring object and the other one sensing the asymmetry in the field caused by the tensile stress.
2. The Prior Art
Attempts with magnetoelastic strip shape measuring have been carried out in several places, and in most cases a number of transducers according to the above-mentioned patent have been used placed in a row across the strip. In all cases, even when transducers of somewhat different types have been used, the individual transducers have been screened by casing of non-magnetic material, and a strong magnetic coupling has been obtained between them. This coupling has resulted in a large zero displacement but, since the transducers have had fixed mutual positions, it has been possible to compensate for this zero displacement.
When measuring the shape of a strip, the stress in a relatively narrow zone at each edge of the strip is of special interest. Using a row of stationary transducers, these edge zones can be sensed only in exceptional cases. At times, therefore, only one single transducer has been used running back and forth on guides across the strip, thus obtaining a consecutive sensing of all strip zones. Besides the obvious disadvantage with a movable arrangement with a flexible lead-in cable, which is in constant motion and is rapidly worn, very sparse information is obtained from each zone at high speeds of the strip. Consequently, a considerable amount of strip with bad shape may have time to pass before any correcting manual or automatic measures can be taken.